(1) Field of the Invention
The present invention generally relates to an electron beam exposure method and an apparatus for carrying out the same. In particular, the present invention is directed to an electron beam exposure method and an apparatus for carrying out the same, in which the step and repeat system and the continuous stage moving system are combined.
(2) Description of the Related Art
The electron beam exposure system is known as a system for forming minute patterns for a large scale integrated circuit (LSI). An example of the electron beam exposure system is the step and repeat system. In the step and repeat system, an electron beam is deflected and irradiated only on an exposure area on a surface of a sample such as a semiconductor wafer placed on a stage kept stationary during exposure. After exposure, the stage is shifted by a predetermined length, and the electron beam is deflected and irradiated on the next exposure area. The above-mentioned procedure is repetitively carried out, whereby desired patterns are depicted on the wafer.
Since the electron beam is not projected onto an area except for exposure areas, there is no serious problem with respect to an exposure time, even when patterns have a high pattern density and a low pattern density. On the other hand, the exposure is not performed during a time when the stage moves. For this reason, a reduced exposure time is not obtainable in a case where patterns having the same pattern density are exposed.
Recently, a continuous stage moving system has been proposed which takes into consideration the above-mentioned viewpoint. See, for example, U.S. Pat. No. 4,362,942. In the proposed continuous stage moving system, patterns are depicted by irradiating the electron beam onto the wafer while the stage is always moved continuously. According to this system, it is possible to obtain the exposure time shorter than that obtainable by the step and repeat system.
However, the proposed continuous stage moving system must select the continuous stage moving speed, depending on the pattern density. FIG. 1 is a a wafer 1 which has areas 2 having relatively low pattern density, and areas such as a test pattern area 3 and an alignment area 4 having extremely high density. The pattern density of the above mentioned wafer 1 changes as shown in the graph of FIG. 2. In order to form patterns on the wafer 1, it is necessary to select an optimum continuous stage moving speed for every pattern position. However, in actuality it is very difficult to do the above. For this reason, in practical operation, a stage continuous moving speed is fixedly selected which makes it possible to depict the area having the maximum pattern density. However, the above has a shortcoming in which the essential feature of the continuous stage moving system, namely, high-speed exposure is lost because of an existence of lower density pattern areas.